Poor type error messages (refer to generated code rather than the higher level problem)
With this module:
module Foo where data Foo a = Foo data Bar a = Bar (Foo a) deriving Eq
the HEAD says:
q.hs:5:0: Non type-variable argument in the constraint: Eq (Foo a) (Use -fglasgow-exts to permit this) In the context: (Eq (Foo a)) While checking the context of an instance declaration In the derived instance: (Eq (Foo a)) => Eq (Bar a)
which isn't really the error message that we want, as it's refering to the generated instance.
- 6 gives the much better:
q.hs:5:5: No instance for (Eq (Foo a)) arising from the 'deriving' clause of a data type declaration at q.hs:5:5 Possible fix: add an instance declaration for (Eq (Foo a)) When deriving the instance for `Eq (Bar a)'
This example is from tcfail046; tcfail169 is similar and tcfail118 is also a related issue.
|Component||Compiler (Type checker)|